Calculate Your House Extension Costs
Your Estimated Extension Cost
Construction Cost
Glazing Package
Kitchen Cost
Professional Fees (10%)
Contingency (10%)
Cost per m²
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How to Use This Calculator
Getting an accurate estimate for your house extension starts with selecting the right options. First, choose your extension type – whether it’s a single storey rear addition perfect for an open-plan kitchen, or a double storey that adds bedrooms above. The calculator accounts for the structural differences between each type.
Next, input your floor area in square metres. Most kitchen extensions are around 20-30m², while larger family rooms often need 40-50m². Your quality of finish dramatically affects costs: budget specs use standard materials and fixtures, while premium finishes include high-end appliances, bespoke joinery, and superior materials throughout.
Location matters significantly – London projects typically cost 20-25% more than elsewhere in the UK due to higher labour rates and material delivery charges. The glazing package option lets you factor in anything from standard double-glazed windows to spectacular bi-fold doors and rooflights that flood your space with natural light.
What Affects Your Extension Cost?
Size and Complexity
Your extension’s footprint directly determines material quantities and labour hours. A straightforward rectangular 25m² addition costs less per square metre than a complex wrap-around design requiring multiple roof valleys and structural steels. Every corner, valley, and change in direction adds to both material waste and construction time.
Structural Requirements
Removing internal walls often requires steel beams, with prices varying from £1,500 for a small opening to £5,000+ for wide spans or two-storey supports. Ground conditions matter too – soft clay or sloping sites need deeper foundations, potentially adding £3,000-£8,000 to your budget. Building Control will specify what’s needed after a site survey.
Specification Choices
Your finish quality determines whether you’re spending £1,500 or £3,000 per square metre. Budget builds use standard sanitaryware and laminate worktops, while premium specifications include underfloor heating, designer tiles, stone worktops, and integrated appliances. Even choosing engineered oak flooring over laminate adds £40-£60 per square metre.
Extension Type Comparison
| Extension Type | Typical Size | Cost Range (Excl. VAT) | Best For |
|---|---|---|---|
| Single Storey Rear | 20-40m² | £40,000-£120,000 | Open-plan living, kitchen-diners |
| Single Storey Side | 15-30m² | £30,000-£90,000 | Additional reception room, office |
| Wrap-Around | 30-50m² | £75,000-£160,000 | Maximum ground floor space |
| Double Storey Rear | 40-80m² | £80,000-£200,000 | Extra bedrooms plus living space |
| Side Infill | 10-20m² | £25,000-£60,000 | Utility rooms, downstairs WC |
Common Questions Answered
Regional Cost Variations
Extension costs vary significantly across the UK. London commands the highest prices at £2,400-£3,200 per m², driven by higher labour rates (£200-£250 daily for skilled trades versus £150-£180 elsewhere), expensive material delivery in congested areas, and stronger demand.
The South East follows closely at £2,200-£2,800 per m², while the Midlands, North West, and Scotland typically see £1,800-£2,400 per m². Wales and the North East offer the most competitive rates at £1,600-£2,200 per m². These figures assume standard specifications – premium finishes add 30-50% regardless of location.
Maximising Your Extension Value
Smart Spending Priorities
Focus your budget where it counts. Quality glazing transforms how your extension feels – large doors connecting to your garden create that wow factor visitors remember. Underfloor heating costs £70-£100 per m² but eliminates radiators, freeing up wall space and providing luxurious comfort.
Don’t skimp on insulation and airtightness. Spending an extra £1,500-£2,500 now on superior insulation (better than Building Regulations minimums) cuts heating bills by £200-£400 annually and makes your space comfortable year-round. Poor thermal performance creates cold spots and condensation problems within months.
Avoiding Expensive Mistakes
Changing your mind mid-project costs dearly. Moving a doorway after plastering starts can cost £800-£1,500. Upgrading from standard to bi-fold doors once the frames are ordered means paying for materials twice. Finalise every detail before construction begins, even if it takes an extra month of planning.
Many homeowners underestimate finishing costs. That £60,000 build estimate might exclude flooring (£2,000-£4,000), decorating (£1,500-£3,000), new furniture for your larger space (£3,000-£8,000), and landscaping the disrupted garden (£2,000-£5,000). Budget an extra 15-20% beyond the builder’s quote for these inevitable extras.
Timeline and Project Management
Before any building starts, expect 8-12 weeks for design and approvals if you need planning permission, or 6-8 weeks just for Building Regulations. Get three detailed quotes from builders – cheapest isn’t always best. Check their previous projects, read reviews, and verify they have proper insurance and liability cover.
During construction, visit regularly but don’t micromanage. Address any concerns immediately rather than letting them fester. Keep a project diary with photos – useful if disputes arise. Hold back that final retention payment until you’re completely satisfied and all Building Control sign-offs are complete.
Future-Proofing Considerations
Think beyond immediate needs. Adding extra sockets and data points during construction costs pennies compared to retrofitting later. Specify wider doorways (900mm instead of 762mm) for accessibility as you age. Consider where you might add solar panels or a home battery in future – reinforcing that area of roof now costs little.
If you might extend further one day, design accordingly. Finishing the side wall properly now makes future connection easier. Running services (water, waste, electrics) slightly beyond your current extension creates simple connection points for that dream conservatory you’ll add in five years.